The present invention relates generally to fiber optic communications, and more specifically to techniques for interfacing optical waveguides to opto-electronic devices such as laser diodes.
Optical fiber has revolutionized the fields of communications and remote sensing. Among current communications applications of optical fiber are telecommunications links, data communications links, high-speed networks, and cable television. Applications of optical fiber to remote sensing include environmental sensing, industrial process control, and smart structures.
Fiber-optic systems typically use a laser diode as an optical source. In fiber-based systems, the light emitted by the laser diode is typically coupled to an optical fiber within the laser diode package, and the optical fiber exits the package via a feedthrough in the side of the package housing. Other than providing coupling between the laser diode and an optical fiber, the laser diode package provides electrical connections to the laser diode, temperature control of the laser diode, and protects the laser diode from a variety of environmental factors that could adversely impact device performance (e.g., moisture, various chemicals, and mechanical shock). The process of coupling the laser diode to the optical fiber is challenging due to very tight mechanical tolerances, resulting in high packaging costs.
Much of the optical fiber used in optical communications and sensing applications is "single-mode" optical fiber, in which the fiber supports only a single guided optical mode. The spatial profile of the guided mode in standard communications-grade single mode fiber using a 1.3 .mu.m wavelength optical source is circular and is approximately 9 .mu.m in diameter. To allow efficient optical coupling from a laser diode to a single-mode fiber, the laser diode must also support only a single guided optical mode. The spatial profile of the guided mode in a typical 1.3 .mu.m wavelength laser diode is an ellipse with axis dimensions on the order of 1 to 2 .mu.m.
Two significant problems are encountered in optical coupling between a single mode optical fiber and a single-mode laser diode. First, the required alignment tolerances between the laser diode and the optical fiber along the axes perpendicular to the direction of light propagation are typically under .+-.1 .mu.m. Second, because the optical mode size of the fiber is typically much larger than that of the laser diode, simply butting the fiber up against the laser diode results in a large mode mismatch loss. Some intermediate optical coupling assembly placed between the fiber and the laser diode is required to reduce the mode mismatch loss and thus achieve high coupling efficiency between the opto-electronic device and the fiber.
Discrete laser diodes with attached fibers (fiber "pigtails") are widely available commercially. The first issue mentioned above, that of stringent alignment tolerances, is typically addressed in commercial package manufacturing through a closed loop alignment process in which the optical coupling efficiency is actively monitored and maximized by moving the fiber end relative to the laser diode. When the fiber is correctly positioned, it is then fixed in place through soldering or the use of an adhesive.
The second issue mentioned above, that of mode mismatch loss, is typically addressed by placing a lens between the fiber and the laser diode to transform the mode size of the light from the laser diode to more closely match that of the fiber. Such a lens is most commonly implemented in commercial laser diode packages by forming a lens at the end of the optical fiber. Numerous techniques for forming a lens at the fiber end have been demonstrated, including mechanical grinding, chemical etching, and melting/pulling the glass. Using lensed fibers, laser-to-fiber coupling losses of less than 3 dB have been widely achieved in commercial laser diode packages.
Other than laser diodes, other single mode waveguided optical devices include semiconductor optical amplifiers (SOAs), integrated-optic electro-optic devices, and passive optical waveguide structures. These other types of devices can be coupled to optical fibers in packages similar to those used with laser diodes. Opto-electronic (OE) technology has advanced to the point that it is now possible to monolithically integrate multiple devices onto a single substrate. Such integrated devices are known as opto-electronic integrated circuits (OEICs). The major materials families in which OEICs are currently being developed are GaAs and InP. For OEICs to be useful as components in fiber communications systems, they must be packaged with optical fiber interfaces.
FIG. 1 schematically shows a package 10 for an OEIC 12 which consists of an array of eight laser diodes 13. The output light from each laser diode is coupled to a respective lensed fiber 15. Each fiber is shown as having a waveguiding core 17 surrounded by a cladding 20. Each of the fiber ends closest to the OEIC is formed with a lens portion 25. The lensed fibers may be fabricated as described above. OEIC 12 and the lensed ends of fibers 15 are located within a housing 27.
In principle, each fiber in the OEIC package can be individually actively positioned and fixed in place, in the same manner as is done with a discrete laser diode. However, for larger numbers of fibers (e.g., 8 or more), the multiple active alignment processes lead to a very time-consuming and expensive packaging process. Additionally, because there is a finite yield associated with each fiber-to-laser diode alignment, the net yield for packages with larger numbers of fibers can become quite low. Furthermore, some OEICs may have waveguides which are on a pitch which is less than one fiber diameter (standard fiber diameter=125 .mu.m). This does not allow fibers to be interfaced to adjacent waveguides.
It has been attempted to get around the need to position each lensed fiber individually by metallizing the fibers and placing (soldering) them in metallized V-grooves on a silicon substrate. The entire array of fibers is then aligned to an array of devices on an OEIC in a single active positioning step. However, this technique has not been effective in achieving high array coupling efficiency due to inadequate fiber positioning accuracy. Factors leading to inadequate accuracy in fiber positioning include variance in fiber cladding diameter, non-concentric positioning of the fiber core relative to the cladding, non-concentric lens position relative to the core center, and imperfect seating of the fiber in the V-groove. These factors add up to give a positioning accuracy in the range of .+-.2 .mu.m. This is significantly more than the required tolerance of under .+-.1 .mu.m, and thus V-groove fiber placement does not provide acceptable performance for OEIC-to-fiber array coupling.